Reed switch having large current carrying capacity



R. A. FUNKE May 2, 1967 REED SWITCH HAVING LARGE CURRENT CARRYINGCAPACITY 2 Sheets-Sheet 1 Filed July 6, 1965 IN V EN TOR. RICHARD A.FUNKE R A. FUNKE 3,317,869 REED SWITCH HAVING LARGE CURRENT CARRYINGCAPACITY May 2, 1967 2 Sheets-$heet 2 Filed July 6, 1965 Patented May 2,1967 3,317,869 REED SWITCH HAVING LARGE CURRENT CARRYING CAPACITYRichard A. Funke, Milwaukee, Wis, assignor to Allen- Bratlley Company,Milwaukee, Wis, a corporation of Wisconsin Filed July 6, 1965, Ser. No.469,507 Claims. (Cl. 335-154) This invention pertains to an electricalswitch which is responsive in relay fashion to an externally appliedmagnetic field.

While not so limited, the switch of this invention can be includedwithin the reed switch classification. Reed switches are generally of asmall construction and essentially consist of magnetic reed memberswhich have terminals at their free ends and overlap at their other endsto perform the dual function of magnetic gap and electrical contacts.These magnetic members are usually enclosed by a non-magnetic material.By applying a sufiicient external magnetic field at the region of thegap, the magnetic members are attracted so as to close the switch. Thewell-known status of this switch in the art obviates the need forfurther discussion.

The switch of the invention is related to the above-described reedswitch classification in that a similar small Switch constructionaccommodates an external magnetic field to close magnetic members; butin the invention, the magnetic gap and the electrical contacts areseparated. As in the reed switch, the switch of the invention ispreferably enclosed. Further significance resides in the fact that theswitch of the inventon has overcome the current capacity limitations ofthe above-described reed switch by means of switch structure which willbe described hereafter. The capacity limitation of the switch isprimarily a function of size with switches comparable in size to theabove-mentioned reed switch handling at least three (3) amps break, five(5) amps carry and thirty amps make at 120 volts All.

An object of this invention is to provide a magnetically responsiveelectric switch which is capable of closing on or making at a maximumcurrent with respect to switch size.

A further object of this invention is to provide a magneticallyresponsive switch with a maximum contact force for switch size; whichcontact force is constant essentially throughout the break or opening ofthe electrical contacts.

A further object of this invention is to provide a magneticallyresponsive switch which incorporates a snap action during the make andbreak of the electrical contacts.

A further object of this invention is to provide a magneticallyresponsive switch which includes a wiping action between the electricalcontacts during the make and break periods.

A still further object of this invention is to provide a magneticallyresponsive switch in which shims are used in combination with a springso as to insure desired switch performance.

A still further object of this invention is to provide a hermeticallysealed switch which is responsive to an external magnetic field and iscapable of closing on or making at a maximum current for its size.

A still further object of this invention is to provide a magneticallyresponsive, hermetically sealed switch in which the magnetic gap isseparated from the gap formed by the contacts to thereby overcomecurrent capacity limitations which are a part of a switch combining saidgaps such as found in reed switches.

A still further object of this invention is to provide a sealed switchutilizing a selected atmosphere in order to prevent sticking between thecontacting members of a magnetic gap within the switch.

These and other objects of this invention will become apparent from thefollowing description of a preferred switch embodiment. It must be notedthat this embodiment is used only for purposes of explanation with thescope of the invention set forth in the appended claims.

FIGURE 1 is a cross-sectional, side View of the switch in itsnormally-open position.

FIGURE 2 is a cross-sectional, side view of the switch in FIGURE 1showing the first step of switch closing, viz the electrical contactsare closed while the magnetic gap remains partially open.

FIGURE 3 is a cross-sectional, side view of the FIG- URE 1 switchshowing the closed position, viz both the electrical contacts and themagnetic gap are closed.

FIGURE 4 shows a modification of the FIGURE 1 switch in which normallyclosed contacts have been added adjacent to the magnetic gap to therebyprovide a form C switch.

Since the construction of the switches shown in FIG- URES 1-3 is thesame and since the construction of the switch in FIGURE 4 is similar tothatrswitch shown in FIGURES 1-3, the following description of theelements making up the switch of the invention will be applicable toeach of the figures and corresponding reference numerals will be used ineach figure.

The elements of this switch are enclosed by tube 1 which is made from anon-magnetic material such as glass, which tube 1 forms seals at eitherend, thereof, about leads 2 and 3-.

Pole piece 8 is secured to the internal end of lead 2 and is made of amagnetic material such as No. 45 Permalloy. Pole piece 8 thus becomesthe element which mechanically connects the armature assembly 9 to thelead 2. Included in armature assembly 9 is a resilient or spring member10 which acts in cantilever fashion to support and bias the armaturepiece 11. In order to control the spring force of spring member 10,inner shim 12 is located between pole piece 8 and spring member 10 whileouter shim 13 is correspondingly located on the opposite side of springmember 10. Each of the shims 12 and 13 are preferably made from asubstantially non-magnetic material such as stainless steel.

The armature piece 11, which is secured to the free end of the springmember 10, is shown in an S-shape so as to complete the magnetic gap 16between pole piece 8 at one end of the armature 11, and at the same timeto provide a surface for movable contact 17 at the other end of armaturepiece 11. In order to enhance the current carrying characteristics ofthe switch at the magnetic gap 16, when this gap is closed as shown inFIGURE 3, the adjacent surfaces 19 of pole piece 8 and armature piece11, which make up the gap 16 may be plated, for example with gold. As inthe case of pole piece 8, the armature piece 11 is made from a magneticmaterial such as No. 45 Permalloy. Completing the elements of armatureassembly 9 is spring support 18 which is a plate adjacent spring memberat the contact 17 end thereof. Support 18 is principally used to aid inthe mechanical union of the armature piece 11 with spring member 10.

The fixed contact 22 is adjacent movable contact 17 and secured to thelead 3. Each of the contacts 17 and 22 uses a material which becomes amatter of design in accordance with the requirements for the switch;while the shape of one or both of the surfaces on the contacts 17 and 22is preferably curved as shown in FIGURES 1-4. Examples of contactmaterials are tungsten, silver and cadmium including combinationsthereof.

To operate the switch of this invention when in the position of FIGURE1, i.e. the normally-open position, an external magnetic field supplied,for example, by a coil or a permanent magnetic field adjacent to andpreferably surrounding the tube 1, is applied at the area of themagnetic gap 16. The resulting magnetomotive force must be sufficient toovercome the spring force of spring member 10 after which the armaturepiece 11 pivots on spring member 10 since armature piece 11 is attractedmagnetically to pole piece 8 at the magnetic gap 16. This spring forceof spring member 10 can be equated to its retractile force or that forcenecessary to be overcome and thereby close the contacts 17 and 22(FIGURE 2) and can then be equated to the contact force between contacts17 and 22 when considering the spring force necessary to fiex springmember 10 and thereby move the armature 11 from the FIGURE 2 to theFIGURE 3 position, i.e. complete closure of the magnetic gap 16.

In order to further control the particular magnetomotive force necessaryto move armature 11, the inner shim 12 may be varied in length so as toselect a desired pivot point for and effective length of the springmember 10. The force to be overcome in spring member 10 is therebyadjusted. Still further control is affected by varying the length ofouter shim 13 to thereby apply necessary restriction to flexure ofspring member 10 which is a function of spring force. The roll of shims12 and 13 will be further discussed below.

As will be seen in FIGURE 2, the steps of switch closing call for thecontacts 17 and 22 to close before the magnetic gap 16 has closed. Thus,switch closing consists of a continuous closing motion comprising twosequential steps, the second of which is shown in FIGURE 3. In orderthat this two step closing along with its purpose will be more clear,indicator lines have been added to the contacts in FIGURES 2 and 3, i.e.indicator line 25 has been added to contact 17 and indicator line 26 hasbeen added to contact 22. The apparent continuity between each indicatorline 25 and 26 in FIGURE 2 illustrates the relative position of thecontact 17 and 22 at the first step of the closing process, i.e. whenthe contact 17 and 22 first meet. The same indicator lines 25 and 26 areshown by FIG- URE 3 as a discontinuous path after the magnetic gap 16has closed.

It is evident that the switch closing motion between the positions shownin FIGURES 2 and 3, which motion constitutes the second step of theclosing process, has caused the movable contact 17 to slide along thesurface of fixed contact 22. This relative contact movement or contactwiping is made available through the resiliency or flexure of springmember 10 and becomes an important feature of this invention. Thisimportance is especially evident when higher currents are to be carriedby the switch since contact wiping establishes a shear as well astension force on welds should they exist between the contacts 17 and 22.As is well-known in the art, these welds become more probable as thecurrent carried by a switch increases.

Once the gap 16 has been closed, the reluctance of the energizingmagnetic circuit is reduced such that less magnetomotive force isnecessary to retain the switch in the closed position of FIGURE 3. Thisphenomenon permits practical application of well-known latchingpermanent magnets (not shown) adjacent the gap 16 so as to hold theswitch closed after removal of the closing mag- 4 netomotive force. Byusing polarized signals, the switch, latched by permanent magnets, canbe opened by inducing a flux which opposes the latching flux to therebyneutralize the necessary latching magnetomotive force.

Opening of the switch shown in FIGURE 3 results from the sufficientremoval of the external magnetic field holding the switch closed andconsequently a reversal of the two step closing process described above.That is, without a sufiicient magnetomotive force, the spring force ofspring member 10 pivots the armature piece 11 so as to first open themagnetic gap 16 and slide contact 17 along contact 22 until the positionof FIGURE 2 is reached; after which, the continued pivotal movement ofarmature piece 11 separates the contact 17 from fixed contact 22. andreturns the switch to the position of FIGURE 1. Control over this springforce of spring member 10 can be obtained by not only varying the lengthof outer shim 13 to thereby vary the pivot point and the degree ofavailable flexure of spring member 10, but also by varying the width andthickness of the spring member 10.

It is important in a switch of this kind to have contact force betweencontacts, e.g. contacts 17 and 22, sufficient to insure low contactresistance when the switch is in the closed position of FIGURE 3. Thisdesired contact force is accomplished through the particular structureof this invention including spring member 10 biasing armature piece 11which connects the separate magnetic gap 16 and the electrical gapsbetween contacts 17 and 22. Specifically, when contact 17 first meetscontact 22 during switch closing, a contact force therebetween isestablished and is proportional to the magnetomotive force at gap 16which force has been transferred to contact 17 through armature piece 11pivoting with spring member 10. Further movement of armature piece 11,viz. closure of gap 16 and sliding of contact 17 upon contact 22,increases the contact force between contacts 17 and 22 proportionally tothat increase in magnetomotive force at gap 16 which is necessary toovercome the spring force of spring member 10. Once the gap 16 closes,the maximum contact force between contacts 17 and 22 has beenestablished.

Since the contact force is a function of spring force, the material usedin spring member 10 becomes a significant design variable as does thelength of shims 12 and 13. For example, the length of inner shim 12varies the effective length of spring member 10 and the pivot point forarmature piece 11 to thereby determine spring force.

'The length of outer shim 13 affects the spring force during the firststep of closing by controlling the flexure of spring member 10 andduring the second step of closing by varying the external force, e.g.magnetomotive force at gap 16, necessary to flex spring member 10 ascontact 17 slides along contact 22. With this available spring forceadjustment, the switch of this invention may be readily designed fordesired as well as maximum contact force, which versatility isconsidered to be a significant contribution.

Besides providing for a contact force whenever contacts 17 and 22 areclosed, the switch construction of this invention furthercauses thecontact force to remain constant essentially throughout the opening orbreak period of the switch. It is to be recalled that the magnetomotiveforce necessary to retain the magnetic gap 16 in a closed condition isboth substantially less than that force which is necessary to close thegap 16 and proportional to the contact force. Upon the initiation ofswitch opening, collapse of that external magnetic field which wasnecessary to close the switch will not influence the magnetic gap 16(and consequently contacts 17 and 22) which remain closed during theinitial and substantial portion of field collapse. However, once thespring force of spring member 10 overcomes the decreasing magnetomotiveforce of the collapsing field, the magnetic gap 16 opens and the contactforce at contacts 17 and 22 diminishes to zero contact force as contacts17 and 22 separate. The period of time during which the contacts 17 and22 remain closed after the opening of the magnetic gap 16, Le. theincrement of time during which armature piece 11 moves from the positionof FIGURE 3 to that of FIGURE 2, is insignificant when compared with thetime during which the switch remains closed while the external fieldcollapses. Thus, it can be said that the contact force remains constantessentially throughout the opening or break period of the switch.

It should be noted that the use of latching magnets to bias the gap 16closed would reduce the relative time for field collapse from theinitiation of switch opening until the contacts 17 and 22 open since themagnetomotive force of such a magnet is more nearly that of themagnetomotive force necessary to overcome the opposing spring force ofspring member 10. Nevertheless, the rela tive time increment duringwhich gap 16 opens is again such that the contact force can be said tobe constant essentially throughout the break period.

The relationship between the magnetomotive force and the contact forceavailable through the cantilever construc tion of armature assembly 9also results in a snap. action or a positive closing which issignificant, particularly under higher current carrying conditions. Byselecting an external magnetic field at gap 16, which is sufiicient tomove the armature piece 11 through the two closing steps shown inFIGURES 2 and 3, the armature piece 11 will move toward closing with asnap motion and will not come to rest until the switch is closed as inFIGURE 3. This snap closing action is very important as it assurespositive closing as well as reducing contact bounce and the undesirableelfects therefrom at closing; the latter being aided in the case of thisinvention by the wiping action between contacts 17 and 22. Snap actionalso exists upon switch opening or the break period as the armaturemoves away from a closed position wtih a snap action. This snap actionthereby aids in the assurance of separation between contacts 17 and 22and is available through the unique armature assembly construction whichprovides necessary spring force in spring member 10.

Achieving snap action upon switch closing requires that themagnetomotive'force exceed the spring force at all times during theclosing of gap 16. As has been pointed out above, the spring force is afunction of desired contact force. Thus, there becomes evident anecessary compatibility of spring and shim construction with magneticgap size and magnetomotive source size in orderto achieve desiredresults. Herein lies a still further contribution of the invention inthat the gap between contacts 17 and 22 may be independently varied inorder to aid in achieving both the desired snap action and contactforce. For example, before sealing the tube 1 around lead 2, the lead 2and predajusted assembly 9 can be moved so that a contact gap size canbe selected which will complement the particular characteristics of thearmature assembly 9. Variation of the necessary magnetomotive force mayalso enter into this contact gap selection. This means simple switchassembly and adjustment are available for a switch structure whichincorporates an advantageous plurality of structural variables.

One method of switch assembly includes sealing one lead, e.g. lead 3,with tube 1. The opposite end of tube 1 remains open. Armature assembly9 is adjusted so as to provide both a selected magnetic gap 16 and aselected spring force in spring member 10 (by means of shims 12 and 13as outlined above). Assembly 9 with lead 2 is then inserted into theopen end of tube 1 so as to establish a contact gap between contacts 17and 22.

Before sealing the tube 1 and lead 2, it is possible to .establishdesired switch performance, e.g. both the desired snap action andcontact force as described above, by adjusting the gap size between thecontacts 17 and 22 through movement of the assembly 9. It is importantthat the contact gap be properly selected since, for example, a contactgap which is too small could prevent snap action while a contact gapwhich is too large could deny closing between contacts 17 and 22. Itshould be noted that contact force would be necessarily increased withthe final closing of magnetic gap 16 should the contacts 17 and 22 betoo close; but this increased contact force exists at the expense ofother desired characteristics such as snap action. Moving the assembly 9in a selected magnetic field there about until snap action occurs, i.e.both the contact gap between contacts 17 and 22 and the magnetic gap 16close, has been used to successfully determine the desired contact gap.The tube 1 is then sealed at the lead 2.

It should be noted that the separation of the magnetic gap 16 from thegap between the electrical contacts 17 and 22 increases current capacityof the resulting switch construction. Specifically, the contacts 17 and22 can be made of material which will accommodate a higher currentwithout interfering with the magnetic conductivity necessary to closethe switch. Furthermore, the use of a separate magnetic gap 16 in theswitch of this invention permits shunting the spring as a currentcarrying conduit when the switch is in its closed position. As has beenpointed out above, the surfaces 19 on pole piece 8 and armature piece 11which are immediately adjacent to the magnetic gap 16 are preferablyplated so as to enhance the electrical conductivity when the magneticgap 16 is closed. The availability of such a shunt permits hithertounavailable flexibility in selecting the material to be used in springmember 10. As has also been pointed out above, the flexibility isimportant when selecting a spring to provide desired contact force. Afurther advantage is illustrated in the fact that without the shunt itwould be necessary to use an alloy such as beryllium-copper in springmember 10 so as to carry the higher current; but the use ofberylliunncopper causes disadvantages such as increased time and expensein manufacture which is not the case when a material such as springsteel is used. Moreover, spring steel has the advantages of higherstrength and fatigue life.

The embodiment of FIGURE 4 is included to illustrate one of the manyvariations to which the switch of FIG- URES 1-3 is susceptible. As isevident, those parts of switch in FIGURE 4 which are common to theswitch structure of FIGURES 1-3 have identical reference numerals. Addedto this previously described structure is contact 31 which is secured tothe armature piece 11 and a contact 32 which is secured to a third lead33 so that a form C switch evolves with lead 2 acting as a common.

The switch of FIGURE 4 operates in the same manner as previouslydescribed for FIGURES 1-3 but with contacts 31 and 32 being separatedwith the pivoting movemerit of armature piece 11 and closing again whenthe armature assembly 9 returns to its normally-open position.

With the exception of leads 2 and 3 which are round in cross-section,the remaining elements, viz pole piece 8, shims 12 and 13 plus thearmature assembly 9, are each esssentially rectangular in cross-section.

The selection of the atmosphere to be found within a sealed switchutilizing an enclosure such as tube 1 depends upon several variablesincluding the particular material to be used in the electric contacts,e.g. 17 and 22. Because of the research and development whichaccompanied the switch of this invention, it can now be concluded thatthe particular atmosphere selected for a sealed switch can affect andthereby prevent sticking between the abutting surfaces at the magneticgap 16. This sticking generally results after repeated closing andopening of the magnetic gap and is especially significant in atmospheresof hydrogen, the inert gases such as helium, neon, argon, krypton andxenon, and in an atmosphere made of a vacuum.

This concept of properly selecting an atmosphere for a sealed switch tothereby prevent sticking at the magnetic gap is exemplified by anatmosphere comprising hydrogen to which nitrogen is added in order thatthe sticking will be eliminated. Additionally, an atmosphere made of avacuum diminished by nitrogen, has satisfactorily prevented magnetic gapsticking. Additives other than nitrogen may be used, such as oxygen,sulfur and compounds thereof; but as with nitrogen, the use and amountof such additives must take into account the effect these atmosphereswill have on switch performance, e.g. the atmospheres arc suppressioncapabilities and tendency to form harmful films on the contact surfaces.In this regard, nitrogen contents of at least about 1% by volume inhydrogen, the inert gases or a vacuum have successfully eliminatedmagnetic gap sticking without substantially affecting switchperformance.

The use of gases such as hydrogen or the inert gases for the atmosphereto be enclosed within tube 1 provides 7 additional advantages in thathigher pressures, eg 1 to 15 p.s.i.g. can be used. These higherpressures result in corresponding increases in breakdown voltages. Onthe other hand, lower pressures can provide desired advantages throughthe less destructive arcing which results.

With modification, the switch of this invention will accommodate stillhigher current capacity; For example, parallel leads with a commonconnection to the switch or a lead made of concentric sections with theinner section capable of accommodating higher current would improvecurrent capacity. Selected materials for parts such as spring member 10and armature 11 will also permit increased current capacity for theswitch.

I claim:

1. A sealed switch which responds to a magnetic field comprising:

(a) magnetic means electrically connected to a first lead,

(b) armature means carrying a first contact means and forming a magneticgap with said magnetic means,

(c) said armature and magnetic means being responsive to said magneticfield at said gap, 7

(d) second contact means associated with said first contact means andelectrically connected to a second lead,

(e) resilient means biasing said armature means to a gap-open position,

(f) said resilient means electrically connected to said first lead,

(g) said magnetic means and armature means acting to electrically shuntsaid resilient means when said gap is closed, and

(h) substantially non-magnetic means interposed between said first leadand said resilient means.

2. The switch of claim 1 wherein said non-magnetic means comprise shimmeans.

3. A sealed switch which responds to a magnetic field comprising:

(a) magnetic means electrically connected to a first lead,

lb) pivoted armature means carrying a first contact means, forming aseparate magnetic gap with said :magnetic means and pivoted at aselected point between but including said first contact and said ((c)said armature and magnetic means being responsive to said magnetic fieldat said gap,

(d) second contact means associated with said first contact means andelectrically connected to a second lead, and

(e) cantilever resilient means with a spring force biasing said armaturemeans in a gap-open position to thereby cause said armature means tomechanically transfer magnetomotive force derived from said field tosaid contacts as a contact force, said magnetomotive force beingproportional to but greater than said spring force.

4. A sealed swtich which responds to a magnetic field comprising:

8 (a) magnetic means electrically connected to a first lead, (b)armature means carrying a first contact means forming a magnetic gapwith said magnetic means and pivoted at a selected point between butincluding said first contact and said gap,

(c) said armature and magnetic means being responsive to said magneticfield at said gap,

(d) second contact means associated with said first contact means andelectrically connected to a second lead,

(e) resilient cantilever means carrying said magnetic armature means ata point separated from said gap to bias said armature means in agap-open position,

(f) said resilient cantilever means connected to said magnetic means byintermediate non-magnetic shim means,

(g) said shim means located adjacent said resilient cantilever means toestablish a pivot point intermediate said first contact and said gap andthereby cause said armature means to mechanically transfer magnetomotiveforce derived from said field to said contacts as a contact force, and

(h) said magnetic means and magnetic armature means acting toelectrically shunt said resilient means when said gap is closed.

5. The switch of claim 4 wherein said resilient means is connected tosaid magnetic armature means so that said first contact will move alongthe surface of said second contact after said contacts have closed.

6. The switch of claim 5 wherein said resilient means carries saidmagnetic armature means so that contact surfaces of said first andsecond contacts meet at an angle.

7. The switch of claim 4 wherein said shim means include means to varythe spring force of said resilient means.

8. A sealed switch which responds to a magnetic field comprising:

(a) magnetic means electrically connected to 'a first lead,

(b) pivoted armature means carrying a first contact means and forming aseparate magnetic gap with said magnetic means,

(c) said armature and magnetic means being responsive to said magneticfield at said gap,

((1) second contact means associated with said first contact means andelectrically connected to a second lead,

(e) cantilever resilient means with a spring force biasing said armaturemeans in a gap-open position to thereby cause said armature means tomechanically transfer magnetomotive force derived from said field tosaid contacts as a contact force, said magnetomotive force beingproportional to but greater than said spring force, and

(f) shim means adjacent said resilient cantilever means so as todetermine an armature pivot point and said spring force.

9. A sealed switch which responds to a magnetic field comprising:

(a) magnetic means electrically connected to a first lead,

(b) pivoted armature means carrying a first contact means, forming aseparate magnetic gap with said magnetic means and pivoted at a selectedpoint between but including said first contact and said (c) saidarmature and magnetic means being responsive to said magnetic field atsaid gap,

(d) second contact means associated with said first contact means andelectrically connected to a second lead, and

(e) cantilever resilient means biasing said armature means in a gap-openposition to thereby cause said armature means to mechanically transfermagneto- 9 motive force derived from said field to said contacts as acontact force.

10. A sealed switch which field comprising:

(a) magnetic means electrically connected to a first lead,

(b) pivoted armature means carrying a first contact means and forming aseparate magnetic gap with said magnetic means,

(c) said armature and magnetic means being responsive to said magneticfield at said gap,

(d) second contact means associated with said first contact means andelectrically connected to a second lead,

(e) cantilever resilient means biasing said armature means in a gap-openposition to thereby cause said armature means to mechanically transfermagnetomotive force derived from said field to said contacts as acontact force, and

responds to a magnetic (f) shim means adjacent said resilient cantilevermeans so as to determine an armature pivot point.

References Eited by the Examiner UNITED STATES PATENTS 1,185,240 5/1916Petersen 200-87 2,481,003 9/1949 Curtis 200-87 X 2,485,024 10/1949 Valeet a1. ZOO-87 2,993,104 7/1961 Zimmer 200-87 X 3,005,072 10/1961 Brown200-87 3,018,353 1/1962 Mitchell 200116 3,070,677 12/1962 Lowry 200873,147,538 9/1964 Perkins 20087 X 5/1965 Juptner 29-155.55

BERNARD A. GILHEANY, Primary Examiner.

I. I. BAKER, Assistant Examiner.

1. A SEALED SWITCH WHICH RESPONDS TO A MAGNETIC FIELD COMPRISING: (A)MAGNETIC MEANS ELECTRICALLY CONNECTED TO A FIRST LEAD, (B) ARMATUREMEANS CARRYING A FIRST CONTACT MEANS AND FORMING A MAGNETIC GAP WITHSAID MAGNETIC MEANS, (C) SAID ARMATURE AND MAGNETIC MEANS BEINGRESPONSIVE TO SAID MAGNETIC FIELD AT SAID GAP, (D) SECOND CONTACT MEANSASSOCIATED WITH SAID FIRST CONTACT MEANS AND ELECTRICALLY CONNECTED TO ASECOND LEAD, (E) RESILIENT MEANS BIASING SAID ARMATURE MEANS TO AGAP-OPEN POSITION, (F) SAID RESILIENT MEANS ELECTRICALLY CONNECTED TOSAID FIRST LEAD, (G) SAID MAGNETIC MEANS AND ARMATURE MEANS ACTING TOELECTRICALLY SHUNT SAID RESILIENT MEANS WHEN SAID GAP IS CLOSED, AND (H)SUBSTANTIALLY NON-MAGNETIC MEANS INTERPOSED BETWEEN SAID FIRST LEAD ANDSAID RESILIENT MEANS.